Transfer truck systems of the type wherein a wheeled frame rides on rails and has at least two drive wheels pressed against a rotating drive shaft in parallel with the rails so that the force for propelling the truck is produced by frictional engagement between the drive wheels and the drive shafts is known. Plural drive wheels in series are provided in order that the truck may support heavy loads. If straight sections of track are connected together by a curved track section, and a series of drive shafts is provided for interconnection the ends of straight drive shafts, the drive wheels oppose each other or lose contact with said series of drive shafts. The present invention is directed to a solution for this problem.